1. Field of the Invention
The present invention relates to low noise narrowband amplifiers for amplifying a radio frequency or microwave signal. Particularly, the invention relates to amplifiers for wireless communication receivers that consume low power.
2. Description of Related Art
Minimization of DC power consumption is useful for prolonging battery life in portable high frequency (i.e., RF and microwave) electronics such as communication receivers. Examples of wireless communication applications that have receivers comprising battery-powered amplifiers include paging, wireless LANs, wireless messaging, wireless modems, cellular telephony, navigation systems, wireless control systems, and wireless security. In such applications, communication receivers may include an antenna to provide the high frequency signals to an amplifier "front-end". However, high frequency signals may be supplied from many different sources, for example by a direct link such as a coaxial cable. A typical high frequency narrowband signal is defined by a center frequency and a narrow band of surrounding frequencies (the "passband"). An advantage of narrow passband is that a narrowband amplifier can be used, which reduces the filter requirements of the system, thereby increasing performance of the overall system and reducing cost.
Lower DC power consumption by an amplifier allows reduction in the size and weight of the required batteries and increases the system operating time before battery replacement or recharge is necessary. Of a system's power consumption, conventional high frequency circuitry consumes a large portion, sometimes more than one-half of the total system power. If the power consumption could be reduced, then battery lifetime could be prolonged.
In addition to low power consumption, a high frequency amplifier should optimize the following qualities: low noise, a close input impedance match, a close output impedance match, high gain in the passband, and unconditional stability in the passband and all other frequencies at which oscillation is possible. Stability is a particularly difficult problem with high performance narrowband low noise amplifiers when operated at lower power consumptions. At higher power consumptions, stability would not be as significant a problem because device impedances usually render the device stable for most terminating impedances at frequencies above the passband. Stability at higher power is mainly due to reduced collector/base feedback capacitance in a bipolar junction transistor (BJT), and reduced drain/gate feedback capacitance in a Field Effect Transistor (FET).
The electrical components for a low DC power amplifier should include high frequency transistors designed for frequencies higher than the center frequency. Although more costly, these high frequency transistors are useful because a transistor's frequency characteristics are derated when operated with low DC power. However, high frequency transistors have disadvantages including a substantial tendency for instability at a wide range of frequencies, particularly frequencies which are more than twice the center frequency.
The noise figure and input match of a high frequency Field Effect Transistor (FET) amplifier with low power consumption can be improved by use of a source inductor coupled between the source and ground. This source inductor is a shunt feedback element. However, stability of the FET amplifier, particularly at high out-of-band frequencies, is a problem. If a Bipolar Junction Transistor (BJT) with an emitter inductor were used in place of the FET with the source inductor, the resulting amplifier would provide similar advantages including improvement in the noise figure and the input match. However, the instability problem is more severe in the BJT amplifier, because the characteristics of a high frequency BJT permit uncontrolled high frequency oscillations to more readily occur and render the amplifier unstable.
It would be an advantage to provide a circuit that can stabilize the high frequency oscillations that would otherwise cause instability in a BJT amplifier with an emitter inductor or a FET amplifier with a source inductor. Such a circuit would allow use of an emitter inductor to improve performance of low DC power consumption/low noise high frequency amplifiers that utilize high frequency bipolar transistors (BJTs). Such a technique could also improve the performance of amplifiers built with FETs.